1. Field of the Invention
This invention relates to improved fibrous sheet materials and methods for forming such materials. More particularly this invention concerns fibrous sheet materials that have been bonded, preferably pattern bonded, with a viscous bonder and adhesive composition and then creped, preferably differentially creped, from a creping surface on which the fibrous sheet material is adhered by the bonder and adhesive. Such fibrous materials have improved tensile strength, softness and bulk; are water dispersible, and are non-blocking.
2. Description of the Prior Art
In the past, there has been extensive activity in the field of papermaking to discover ways of imparting softness to paper webs without degrading their strength. Paper webs are conventionally softened by working them in different ways, such as by creping them from a surface, usually a drying surface with a creping blade. Such a process disrupts and breaks many of the inter-fiber bonds in the paper web which are formed during drying. However, these inter-fiber bonds are the principal source of strength in an ordinary paper web. Very little strength results from the physical entanglement of the fibers since papermaking fibers have an extremely short length, generally on the order of 1/16 inches or less.
The softness of creped, fibrous sheets has been increased by chemically impeding or preventing the inter-fiber bonds while the strength of the sheet has been maintained by adding bonding materials to the sheet. The chemicals which usually interfere with the inter-fiber bondings also tend to prevent proper adhesion of the fibrous sheet to the creping surface and therefore interfere with creping. To overcome this undesirable affect, creping adhesives have been added to the sheet or the creping surface to obtain proper adhesion between the fibrous sheet and the creping surface. The bonder and/or adhesive materials that are added to the sheet can be uniformly applied across the whole sheet or printed onto the sheet in a specific pattern.
In the production of soft, creped sheets for tissue products it is desirable that the chemicals added to the sheet for adhesion to the creping surface and for bonding of the fibers to strengthen the sheet be water dispersible in order that the tissue will disperse when discarded in water after use. Another requirement for creped, fibrous sheets is known as the non-blocking property. Soft, creped sheets suitable for tissues and similar products are usually rolled and it is necessary that the sheet unravel from the roll without sticking together or otherwise interfere with unraveling. Interference with unraveling when caused by the chemicals added for fiber bonding or adhesion to the creping surface is called a blocking effect. Therefore, a desirable creping adhesive and fibrous sheet bonder for use on soft tissue type products must adequately adhere the sheet to the creping surface, must provide adequate bonding strength to the fibrous sheet, must be water dispersible, must product a soft sheet and must set sufficiently in the papermaking process to be non-blocking when the sheet is rolled. All of these properties were not attainable with a single chemical.